Lithographic production is useful for integrated circuits, masks, reticles, flat panel displays, micro-mechanical or micro-optical devices and packaging devices, e.g. lead frames and MCM's. Lithographic production may involve an optical system to image a master pattern from a computer-controlled reticle onto a workpiece. A suitable workpiece may comprise a layer sensitive to electromagnetic radiation, for example visible or non-visible light. An example of such a system is described in WO 9945435 with the same inventor and applicant as the present invention.
Said computer controlled reticle may for instance be a Spatial Light Modulator (SLM) comprising a one or two dimensional array or matrix of reflective movable micro mirrors; a one or two dimensional array or matrix of transmissive LCD crystals; or other similar programmable one or two dimensional arrays based on gratings effects, interference effects or mechanical elements, e.g. shutters.
Generally speaking pattern quality may be improved by multipass writing. However, there are several different aspects of the pattern quality that are improved by multipass writing, but not necessarily at the same time. First, it is possible to create a finer address grid in several passes than in one single pass. Second, multiple passes with offset grid can remove the grid effects due to the finite pixel size.
Third, random errors (such as artifacts in the light path, noise in the exposure dose, misplacement of beam or field used for imaging) are statistically reduced by multiple passes, e.g., four passes reduces the effects of random dose errors by a factor of two (square root of four). Fourth, systematic errors (such as dose fall of in corners of the image to be written, distortion and focal plane curvature) can be reduced by offset between the writing fields. Fifth, with multiple writing passes it is possible to better correct for bad pixels. Sixth, many multipass schemes give a softening of the edges, and retention of edge acuity is a desirable property of a multipass scheme.
Different rasterizing multipass schemes can be devised, but it is a problem to find schemes that simultaneously give improvements in all six aspects mentioned above.
FIG. 3a illustrates a known method for creating a virtual grid. In FIG. 3a it is shown an array of seven lines and 5 columns of pixels. Pixels in the two leftmost columns are set to a maximum grayscale value. Pixels in the two rightmost columns are set to minimum grayscale value. Pixels in a middle column are set to an intermediate grayscale value. FIG. 3a is an example of analog modulation of feature edge pixels 301 in a single pass in order to create a virtual grid. All pixels in said middle column are set to the same value.
FIG. 3b illustrates another known method for creating a virtual grid. In this method four writing passes 305 are written with binary doses (e.g. 100%, 50%, 25%, 12,5%). All pixels in one single pass are set to equal grayscale values. The virtual grid is created by turning on a column of feature edge pixels in at least one writing pass, in FIG. 3b columns of feature edge pixels are turned on in the top writing pass and the second writing pass from bottom.
FIG. 3c illustrates yet another known method for creating a virtual grid. In this method all four writing passes 305 are written with the same dose. In at least one pass a column 304 of feature edge pixels are turned on, illustrated in FIG. 3c to be the bottom writing pass and the second writing pass from the bottom.
FIG. 4a illustrates another known method for creating a virtual grid. In this method four 401 passes are written offset with equal dose. The different writing passes are illustrated in FIG. 4a to be offset relative to an origin 402. By turning on edge pixels 403 in only some passes an edge of a feature to be written can be fine positioned.
FIG. 4b illustrates still another known method for creating a virtual grid. This method utilizes a combination of the analog modulation of feature edge pixels with offset passes which gives a different analog value 404 in each pass.
FIG. 5a illustrates a writing grid of four pixels in a single pass writing strategy. A reference mark denoted with 501 is arranged in middle of the grid.
FIG. 5b illustrates a known method for offsetting different writing passes. Here four passes are used where two of them are offset relative to the other two by a distance defined by a half pixel size in two orthogonal directions parallel with the pixel grid. By offsetting different writing passes in a multiple writing strategy different imaging defects can more or less effectively be hidden.
FIG. 5c illustrates another known method for offsetting different writing passes in order to hide the grid. In this embodiment all writing passes are offset relative to each other. One writing pass is offset in a first direction only, another writing pass is offset in a second direction, orthogonal to said first direction, and one writing pass is offset in sad first and sad second direction simultaneously. The offset in said first and said second direction is illustrated to be half a pixel size.
It is in a general sense possible to make pattern fidelity better by increasing the number of passes, but the cost is high. Doubling the number of passes doubles the capital and operating cost of the pattern generator per produced workpiece and can in many cases be economically impossible.
In general, computer controlled reticles may be used for the formation of images in a variety of ways. These reticles, such as an SLM, include many modulating elements or pixels, in some instances, a million or more pixels.
In WO 99/45440, with one common inventor to the present application, is described a pattern generator with improved address resolution. In said application the pixels can be set in a number of states, larger than two, with one type of pixel map inside pattern features, another type of pixel map outside pattern features and intermediate pixel maps at a boundary of pattern features. The intermediate pixel map is generated in dependence of the placement of the boundary in a grid finer than that of pixels of the SLM projected onto the workpiece.
Due to the fact that the line width and a space between two lines in the pattern to be printed on the wafer are very small, it puts a lot of demands on the printing method and the apparatus using said method. Using an SLM, which provides for a too coarse address grid may limit the resolution and accuracy available for their use in optical imaging; e.g., the production of printed patterns on a workpiece may be limited as to its line widths and accuracy.
Therefore, there is a need in the art for a method, which further fine-adjust the position of the element's edge in the image created on the workpiece. There is also a need in the art for an improved effectiveness of multipass averaging, i.e., reduced number of passes giving an improved fine-adjustment of a feature edge.